The Yellow Fever virus is endemic, that is, continuously present with low levels of infection in some tropical areas of Africa and the Americas, where it regularly amplifies into epidemics. Other parts of the world, including coastal regions of South America, the Caribbean islands, and Central and North America, are infested with the mosquito vector capable of transmitting the virus and are therefore considered at risk for yellow fever epidemics (World Health Organization Fact Sheet No. 100, revised December, 2001).
For example, in Africa alone, thirty-three countries with a combined population of 508 million, are at risk (Id.). Each year, the World Health Organization (WHO) estimates there are 200,000 cases of yellow fever, with 30,000 deaths (Id.). Travel to these tropical regions also is believed to result in a small number of imported cases in countries generally free of yellow fever. Although yellow fever cases have not been reported in Asia, “this region is at risk because the appropriate primates and mosquitoes are present” (Id.).
The Yellow Fever (YF) virus is in the genus Flavivirus, in the family Flaviviridae. In the so-called “jungle” or “sylvan cycle”, the YF virus is enzootic, maintained and transmitted by canopy breeding mosquitoes to monkeys in the rainforests. The “urban cycle” begins when humans become infected by entering the rainforests and are bitten by YF-infected mosquitoes. The “urban cycle”continues with peridomestic transmission from humans to mosquitoes and thence to other humans, and can result in yellow fever epidemics in villages and cities. Illness ranges in severity from a self-limited febrile illness to severe hepatitis and fatal hemorrhagic disease.
Unvaccinated humans, including both native people and travelers to YF endemic areas are at significant risk of YF infection when occupational and other activities bring them in contact with infected mosquitoes in the sylvan cycle or the urban cycle.
Patients with yellow fever may be viremic, i.e., have virus in their blood, for 3 to 6 days during the early phase of illness. This phase may be followed by a short period of symptom remission.
The toxic phase develops as the fever returns, with clinical symptoms including, for example, high fever and nausea, hemorrhagic symptoms, including hematemesis (black vomit), epistaxis (nose bleed), gum bleeding, and petechial and purpuric hemorrhages (bruising). Deepening jaundice and proteinuria frequently occur in severe cases.
In the late stages of disease, patients can develop hypotension, shock, metabolic acidosis, acute tubular necrosis, myocardial dysfunction, and cardiac arrhythmia. Confusion, seizures, and coma can also occur, as well as complications such as secondary bacterial infections and kidney failure.
There is no specific treatment for yellow fever. Steps to prevent yellow fever include use of insect repellent, protective clothing, and vaccination with the available, but risky attenuated vaccine.
Live, attenuated vaccines produced from the 17D substrain, are available, but adverse events associated with the attenuated vaccine can lead to a severe infection with the live 17D virus, and serious and fatal adverse neurotropic and viscerotropic events, the latter resembling the severe infection by the wild-type YF virus. Thus there is a need for a safer, inactivated, non-replicating vaccine that will elicit a neutralizing antibody response while eliminating the potential for neurotropic and viscerotropic adverse events.
Thus, there is an on-going need for an effective, inactivated, “killed” or non-replicating vaccine in order to avoid the potential for neurotropic and viscerotropic adverse events associated with the currently available attenuated YF 17D vaccine. Further, there is a need for an improved vaccine produced in Vero cells without animal-derived proteins, a vaccine that can be safely used for persons for whom the live vaccine is contraindicated or for whom warnings appear on the label. Such individuals include immuno-suppressed persons, persons with thymic disease, egg-allergic, young infants, and the elderly.
A problem with any potential inactivated virus is that it may need to be delivered at a higher titer than the existing live attenuated vaccines, because the latter can expand antigenic mass during cycles of replication in the host whereas an inactivated vaccine contains a fixed dose of antigen. Therefore, in order to develop a sufficiently potent inactivated vaccine, it is desirable to modify the YF virus in order to produce a high yield of virus in the conditioned medium (also called supernatant fluid) of a cell culture. It is highly desirable to use the attenuated 17D vaccine strain for vaccine manufacturing, since the 17D strain can be manipulated at a lower level of biocontainment than the wild-type virulent YF virus. However, the attenuated 17D vaccine strain yields in cell culture are inherently lower than yields of wild-type virus. For these reasons, modifications of the 17D vaccine strain to achieve higher yields in cell culture used for vaccine production would be useful.